1. Field of the Invention
This invention relates generally to coating compositions and coated substrates. In particular, the invention includes biomimetic peptide-containing coating compositions and magnesium alloy substrates having applied thereto, or deposited thereon, the biomimetic peptide-containing compositions, wherein the coated magnesium alloy substrates are useful for tissue and bone repair and regeneration, such as but not limited to, as medical implant devices in orthopedic, craniofacial, dental and cardiovascular surgeries.
2. Background Information
Biomedical implant devices are known in the art and are commonly used in the practice of various surgeries, such as, orthopedic, dental, craniofacial and cardiovascular implant surgeries. These devices may be used for various purposes, such as but not limited to, tissue and bone regeneration, and drug or biomolecule delivery. Furthermore, stents are also implanted into a body of a patient to support lumens, for example, coronary arteries. Implant devices include, but are not limited to, scaffolds, such as plates and screws. Biomaterials for the construction of implant devices are typically chosen based on their ability to withstand cyclic load-bearing and compatibility with the physiological environment of a human body. Many of these implant devices are traditionally constructed of polymer or metal. These materials of construction exhibit good biomechanical properties. Metallic biomaterials, in particular, have appropriate properties such as high strength, ductility, fracture toughness, hardness, corrosion resistance, formability, and biocompatibility to make them attractive for most load bearing applications. Polymers, such as polyhydroxy acids, polylactic acid (PLA), polyglycolic acid (PGA), and the like, are known as conventional biomaterials, however, in some instances the strength and ductility exhibited by polymers is not as attractive as that demonstrated by metallic biomaterials. For example, it is known to use stainless steel or titanium biomedical implants for clinical applications which require load-bearing capacities.
Metallic and polymer biomaterials are not biodegradable and therefore, other biomaterials need to be used wherein there is an interest to provide a biodegradable implant device such that the device is capable of degrading over a period of time, e.g., by dissolving in the physiological environment, and surgery is not required for remove when there is no longer a medical need for the implant device. Magnesium is potentially attractive as a biomaterial because it is very lightweight, has a density similar to cortical bone, has an elastic modulus close to natural bone, is essential to human metabolism, is a cofactor for many enzymes, and stabilizes the structures of DNA and RNA. Magnesium-based implants may be degradable in-vivo through simple corrosion and exhibit mechanical properties similar to native bone.
There are, however, disadvantages associated with magnesium which have restricted its use in medical applications. For example, magnesium is very reactive in nature and is susceptible to rapid corrosion as opposed to gradual degradation, particularly, in high chloride environments such as those created by human body fluids and blood plasma, and in aqueous solutions having a pH of 11 or less. The physiological pH is typically in the range of 7.4 to 7.6. During magnesium corrosion, a local pH increase as well as hydrogen liberation may ensue. If the evolution of gas is too rapid, it cannot be absorbed by the body and poses a significant concern for medical applications.
In order for magnesium to be considered an acceptable biomaterial for tissue and bone replacement and regeneration, improvement of its corrosion resistance is needed. Thus, there have been methods developed in the art for the purpose of improving the corrosion resistance of magnesium. Known methods include element alloying and surface modification or coating.
Traditional surface modification methods include electrochemical plating, chemical conversion, anodizing, gas phase deposition, and organic coatings. An effective and mature chemical conversion process known in the art is based on using a chromate bath. However, use the chromate bath is limited due to its high toxicity. The application of plating and anodizing techniques are also limited by their dependence on toxic heavy metal ions and their adverse effects on fatigue properties.
Known magnesium alloy coatings typically include the use of ceramic, chitosan, and various forms of calcium phosphate (CaP). Application of a CaP coating by ion-beam-assisted deposition or various types of electrochemical and chemical treatments can provide a reduced corrosion rate. However, the crystal structure, chemical composition, coating morphology, and the measured degradation rates can exhibit variability. In addition, even though initially good cell adhesion and spreading of CaP coatings has been demonstrated, cell viability has shown to be compromised at longer time periods, which is likely due to the poor corrosion protection of CaP alone.
Other known coatings have also demonstrated variable results in enhancing corrosion resistance or a lack bioactive properties necessary for controlling cellular behavior. Further, coupling of the coatings to magnesium-based alloys as well as non-resorbable metals, such as titanium, have generally produced low coating affinity/low bonding strength on the alloy itself.
Further, in considering biologically-derived surface coatings or modifiers, such as peptides, for use in coating magnesium alloy, it is known that bone and dentin are examples of mineralized tissue, which are unique, hierarchical nanocomposites and can include about 70% by weight carbonated apatite, 20-25% by weight organic matrix, and 5-10/% by weight water. Mineralized collagen fibrils are the major organic components, e.g., building blocks, of these tissues. Other non-collagenous proteins (NCPs) and glycoproteins account for less than about 10% of the total organic content and contribute to the regulation of mineralization, cell signaling and mechanical performance of the tissue. It has been shown that the mineral component in mineralized collagen fibrils has almost two times greater strain than geologic or synthetic apatite, and the organic component is significantly stiffer than non-mineralized collagen. Furthermore, the interlaced structure of the mineralized collagen fibrils provide a complex organization and unique mechanical properties.
NCPs are involved in collagen mineralization and a characteristic of NCPs is the disproportionately large percentage of acidic amino acids such as Asp, Glu and Ser(P). For example, the major NCP in dentin is phosphophoryn (DPP). DPP includes primarily Ser-Ser-Asp repeat motifs with more than 90% of serines phosphorylated.
Protein phosphorylation is one of the most common post-translational modifications. However, the vast majority of phosphorylated proteins contain only a small amount of phosphorylation sites adjacent to kinase-specific recognition motifs. The precise phosphorylation mechanisms of the highly phosphorylated proteins from the mineralized tissues are not completely understood in the art. It has been proposed that casein kinases (CK) 1 and 2 phosphorylate DPP intracellularly in the endoplasmic reticulum. According to certain hypotheses, phosphorylation occurs via a chain or hierarchical reaction wherein one phosphorylated serine becomes a part of the CK recognition site which leads to subsequent phosphorylation of new serines. CK transfer γ-phosphate of ATP (or GTP) to the hydroxyl group of serine or threonine, or to the phenolic hydroxyl on tyrosine residues in proteins.
A number of peptides mimicking NCPs have been synthesized. However, the known syntheses for preparing bio-inspired peptides has limitations. For example, introducing any single phosphorylated amino-acids during peptide synthesis leads to a significant decrease in yield, thereby limiting the total number of phosphorylated amino acids that can be added to a peptide.
Thus, there is a need in the art for magnesium alloy-containing composites for tissue and bone replacement and regeneration, such as tissue and bone implants, which exhibit the non-toxicity and mechanical properties that are desired while demonstrating reduced rate of corrosion when exposed to physiological conditions. It is an objective of this invention to design and develop novel biomimetic peptide-containing compositions for application to or deposition on magnesium alloy such as to form a coating on a surface of the magnesium alloy. It is desirable for these biologically-derived coatings and coated magnesium alloys to be effective for magnesium corrosion control, calcium phosphate (CaP) deposition, and cell signaling capabilities to enhance tissue regeneration.